It has heretofore been required to obtain an optical glass having a high viscosity at melting and a slow crystal growth rate at a temperature below the liquid phase temperature and which is suitable for a direct press molding from a continuous melting furnace, from inexpensive glass raw materials. Also, an optical glass in which coloring and scratches do not almost generate during polish processing and having an excellent chemical durability has been required.
To achieve the above object, a silicate glass is advantageous as compared with a borate glass. However, the silicate glass containing a large amount of La.sub.2 O.sub.3 has disadvantages such that the stability is poor and a dissolution temperature is high. For these reasons, many conventional optical glasses having the above-described optical constant ranges are glasses that the SiO.sub.2 content is small and the B.sub.2 O.sub.3 content is large or that injurious components such as CdO or ThO.sub.2 are contained. However, a glass composition containing a large amount of B.sub.2 O.sub.3 is difficult to produce a uniform optical glass with a good yield due to a low viscosity at melting, resulting in volitization of some components. Even if an optical glass can be produced, such a glass does not have a sufficient chemical durability.
On the other hand, in the case that B.sub.2 O.sub.3 as a glass forming material is substituted with SiO.sub.2, the liquid phase temperature arises and the solubility is reduced as the amount of substitution increases. To improve these disadvantages, an optical glass containing Nb.sub.2 O.sub.5 or Ta.sub.2 O as an essential component has been proposed as described in, for example, Japanese Patent Publication No. 7124/68 and Japanese Patent Application (OPI) No. 88106/73. Such a glass, however, has disadvantages that the raw materials are expensive and where the amount of B.sub.2 O.sub.3 is decreased to a certain degree, the melting temperature cannot be reduced.